the gas conversions I have seen on diesels are a booster to the diesel system, rather than running on one or the other. The gas is fed into the air intake system which vastly enhances combustion and reduces Nox etc

Diesel burns more efficently with gas added to it so it improves power and economey etc.

Its a little like the hydrogen system for diesels where they just feed some gas into the maifold and it improves combustion, enhancing performance and turbo response, cleaner longer lasting oil etc etc